Such fiber-optic amplifiers are known from, e.g., Laming, R. I., et al, "High Sensitivity Optical Pre-Amplifier at 10 Gbit/s Employing a Low Noise Composite EDFA with 46 dB Gain", Optical Amplifiers and Applications, Technical Digest 1992, (Optical Society of America, Washington D.C. 1992), Vol. 17, pages 58 to 61.
A two-stage fiber-optic amplifier is shown there whose amplifying sections of optical fiber are doped with erbium ions (Er.sup.3+).
A signal having a wavelength of 1536 nm is amplified in two amplifier stages. The erbium ions are excited by a pump laser which emits pump light at a wavelength of 980 nm. After the first amplifier stage, the pump light is guided past an optical isolator by means of two wavelength-selective couplers. The optical oscillator blocks the light caused by amplified spontaneous emission (ASE) and travelling opposite to the direction of propagation of the signal. Pump light and signal then pass through the second amplifier stage, after which the amplified signal is available, which is fed to a receiver.
One disadvantage of this configuration is that in the receiver, particularly at high signal powers, noise is present in addition to the noise caused by spontaneous emission.